Xiang Ma, Chunyang Cui, Shengrong Xie, Xi Jiang, Jiaming Chang, Yantong Zhao
Mudstone represents a typical water-sensitive aggregate that absorbs free water from cement paste during the setting process, leading to progressive softening and disintegration. Simultaneously, mud film at the cement–mudstone interface severely weakens the interfacial transition zone (ITZ) and the macroscopic mechanical performance of mudstone aggregate concrete (MAC). Conventional waterproof layers effectively repel free water from cement paste but often prevent the paste from bonding with the aggregates. In this study, a composite waterproof adhesive layer (WAL) is designed to overcome this limitation through synergistic reaction mechanisms. The enhancement behaviors are investigated through uniaxial compressive strength (UCS) tests, interfacial direct shear tests, contact angle measurements, and microstructural characterizations (SEM–EDS, AFM, XRD, and FTIR). PFC 2D discrete element simulations are employed to understand how enhancement in the micromechanical properties of the ITZ governs the macroscopic strength improvement of MAC. The results demonstrate that the UCS of MAC increases by 40.4% and the interfacial cohesion improves by 61.7% after WAL modification. Notably, the macroscopic failure mode transforms from a matrix–ITZ–matrix to a matrix–aggregate–matrix shear failure. Microstructural analyses reveal that the WAL establishes a continuous, dense, and high-strength ITZ at the mudstone–cement interface by water resistance, hydration-product filling, and mechanical interlocking. Numerical simulations indicate that WAL can effectively suppress premature cracking within the ITZ and promote stress transfer into the aggregates efficiently, resulting in a cooperative aggregate–matrix load-bearing structure. This study clarifies the role of ITZ modification in enhancing the strength of MAC and provides a new material design for grouting reinforcement and high-value reutilization of water-sensitive mudstone in underground engineering.